At college, 70% of the students studied Maths, 75% studied English, 85% studied French and 80% studied German. What percentage at least must have studied all 4?

A card player holds 13 cards of four suits, of which seven are black and six are red. There are twice as many hearts as clubs and twice as many diamonds as hearts. How many spades does he hold?

At each stage the black dot moves three corners clockwise and the white dot moves four corners anticlockwise. After how many stages will both dots be together in the same corner?